Since there is no software available that uses hooks, deinstall plug_uddeim_hooks. That should fix your problem.
If you need hooks, you could try following:
In \plugins\system\uddeim_hooks\uddeim_hooks.php
add following line
if (array_key_exists($event, $this->callbacks))
right before
if (count($this->callbacks["$event"])) {
Btw it is only a "notice" not a warning or error, so no problem at all.
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer